1. Field of the Invention
The present invention relates to a plasma process apparatus and, more particularly, a plasma CVD apparatus for using plasma to process substrates such as semiconductor wafers and LCD substrates.
2. Description of the Related Art
In a plasma process apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. Hei 3-79025, a flat induction coil is arranged outside a process chamber by an insulating shield, and a radio frequency power supply is connected to the induction coil. The induction coil produces induction electromagnetic field to induce the movement of electrons and develop plasma.
In another plasma process apparatus disclosed in European Patent Publication No. 0.379.828, radio frequency is added to a vortex or spiral induction coil to generate plasma in the process chamber. Plasma is closed in the process chamber by bipolar magnetic field.
FIG. 11 shows the profile of film thickness of a semiconductor film formed on a semiconductor wafer by a conventional plasma CVD apparatus in which an RF induction coil is used. The RF induction coil 2 has an outer diameter substantially the same as that of the wafer W and is arranged coaxial to the wafer W. Hatched portion between the coil 2 and the wafer W in FIG. 11 represents the profile of film thickness of the film formed.
When the conventional apparatus is used, the film thickness becomes small at the center and the outer rim portion of the wafer W while it becomes large between the center and the outer rim portion thereof, as shown in FIG. 11. This unevenness of film thickness cannot be ignored as the diameter of the wafer W becomes larger and larger, to 6, 8 and 12 inches. In a substrate like the LCD (liquid crystal display) substrate having a large area of 500 mm×600 mm or 550 mm.×650 mm, the unevenness of film thickness of a formed film provides a problem.
The RF induction coil is arranged adjacent to a quartz-made dielectric window which is a part of wall for defining the process chamber. In the case of the plasma process, particularly the film forming process, by-products easily adhere to the inner face of the window. When the by-products are conductive, the induction electromagnetic field generated by the RF induction coil is disturbed and plasma thus formed becomes sometimes uneven.